Stress birefringence compensation in polarizing beamsplitters and systems using same

ABSTRACT

An optical system is provided. The optical system comprises a first polarizing beamsplitter (PBS), a first reflecting image-forming device, and a first quarter-wave retarding element. An optical system is also provided that comprises an image-forming device, a reflective polarizing layer, at least a first birefringent element disposed on an optical path between the image-forming device and the polarizing layer, and a quarter-wave retarding element. Further provided is an optical system comprising a color combiner unit, at least two polarizing beamsplitters (PBSs), and a first quarter-wave retarding element. The present application also provides methods of compensating for birefringence in an image projection system.

RELATED PATENT APPLICATION

This patent application is a divisional of U.S. patent application Ser.No. 11/088,153, which was filed on Mar. 23, 2005, and which isincorporated herein by reference in its entirety.

FIELD

The invention relates to optical systems and more particularly to theoptical systems that use polarizing beamsplitters for separating orcombining light beams in different polarization states.

BACKGROUND

The function of a polarizing beamsplitter (PBS) is to reflect light inone polarization state and to transmit light in the orthogonalpolarization state. Consequently, PBSs find widespread use in opticalsystems that rely on the polarization of the light. An example of onesuch system is an image projection system that uses a reflective liquidcrystal display (LCD) panel for modulating an illumination light beam: apolarized illumination light beam is directed to the LCD panel, forexample by reflection in the PBS. The light beam is spatially modulatedby the LCD panel so that the reflected beam contains some unmodulatedlight in the polarization state of the illumination beam and somemodulated light in the orthogonal polarization state. The unmodulated,non-image light is reflected by the PBS and the modulated, image light,which contains the desired image, is transmitted through the PBS.

Thus, the PBS separates the image light from the non-image light and theimage light can then be projected to a screen for viewing by a user.

Different types of PBS may be used: projection systems have beenreported using MacNeille PBSs, which rely on a stack of quarter wavefilms of isotropic material oriented at Brewer's angle for one of thepolarization states, and using a Cartesian multilayer optical film (MOF)PBS, which uses a stack of alternating isotropic and birefringentpolymer materials. The Cartesian MOF PBS is capable of operating atlower f-numbers and with higher contrast and transmission than theMacNeille PBS.

PBSs are often formed as a polarizing layer sandwiched between thehypotenuses of two right-angled, glass prisms. If, however, there is anybirefringent retardation in the glass prism lying between the polarizinglayer and the imager panel, the contrast provided by the PBS can bereduced because the nominally s-polarized illumination light reflectedfrom the polarizing surface is rotated to being partially p-polarizedwhen incident at the imager panel. This causes leakage of light afterreflection from the imager panel, resulting in an increase in the levelof brightness in the dark state and, therefore, a reduction in thecontrast. Birefringent retardation in the glass prism may result from anumber of different causes, for example, mechanical stresses induced inthe PBS components while assembling the PBS, or stresses induced by thePBS fixture or by thermal expansion in the PBS when subjected to theintense illumination light beam. In addition, if hardware to mount animaging device is attached to the input prism of a PBS, this willusually result in dramatic contrast reductions, over at least someregions of the PBS, due to resulting stresses in the glass.

In an effort to overcome this problem, a significant amount of work wasdone by glass manufacturers to make glass with a low stress-opticcoefficient (SOC) that develops very little birefringence in response tomechanical stress. PBH56 and SF57 are example glasses of this type, madeby Ohara and Schott respectively. Such glasses contain lead insignificant quantities: both PBH56 and SF57 contain more than 70% leadoxide by weight. These low SOC glasses are, therefore, notenvironmentally desirable materials and are also expensive and difficultto process. Additionally, the low SOC glasses have a high refractiveindex, in excess of 1.8, which may lead to optical inefficiencies oraberrations when matching to the lower refractive index polarizinglayers.

SUMMARY

One embodiment of the invention is directed to an optical unit thatcomprises a polarizing beamsplitter (PBS). The PBS has a first coverhaving a first surface and a second surface for transmitting a lightbeam and a second cover having at least a first surface for transmittinga light beam. The second cover is arranged with its first surfaceopposing the first surface of the first cover. A reflective polarizinglayer is disposed between the first surfaces of the first and secondcovers. A quarter-wave retarding element is disposed proximate thesecond surface of the first cover. The quarter-wave retarding element isaligned substantially to maximize compensation for birefringence of thefirst cover for light that is double-passed through the first cover,between the first and second surfaces.

Another embodiment of the invention is directed to an optical systemthat comprises a first polarizing beamsplitter (PBS) having a reflectivepolarizing layer disposed between first and second covers. The firstcover is formed of a material having a stress optic coefficient greaterthan 0.1×10⁻⁶ mm² N⁻¹. A first reflecting image-forming device faces afirst surface of the first cover so that light passes through the firstcover from the reflective polarizing layer of the first PBS to the firstreflecting image-forming device and is reflected from the reflectingimage-forming device through the first cover to the reflectivepolarizing layer of the first PBS, the light being subject to stressbirefringence in the first cover and to residual birefringence in thereflecting image-forming device. A first quarter-wave retarding elementis disposed to retard the light passing between the first surface of thefirst cover and the reflecting image-forming device.

Another embodiment of the invention is directed to an optical system,that has an image-forming device and a reflective polarizing layerdisposed to reflect light in a selected polarization state to or fromthe image-forming device. A birefringent element is disposed on anoptical path between the image-forming device and the polarizing layer.The birefringent element has a face substantially perpendicular to theoptical path. The birefringence of the birefringent element isnon-uniform so that values of birefringent retardation for light passingthrough different portions of the face are different. A quarter-waveretarding element is disposed between the birefringent element and theimage-forming device. The quarter-wave retarding element is oriented toat least partially compensate retardation of light on a round tripbetween the reflective polarizing layer and the image-forming element.

Another embodiment of the invention is directed to an optical systemthat comprises a color combiner unit having at least two entrance facesassociated with different color bands and at least two polarizingbeamsplitters (PBSs) operatively disposed relative to the at least twoentrance faces respectively. At least a first PBS of the at least twoPBSs comprises a first reflective polarizing layer disposed betweenrespective first surfaces of first and second covers. The second coveris positioned between the reflective polarizing layer and the colorcombiner unit. The first cover is formed of a transparent materialhaving a stress-optic coefficient (SOC) greater than 0.1×10⁻⁶ mm² N⁻¹.The first PBS defines a first plane of reflection. The first cover ofthe first PBS lies between a quarter-wave retarding element and thereflective polarizing layer. The quarter-wave retarding element has afast axis oriented non-parallel and non-perpendicular to the first planeof reflection.

Another embodiment of the invention is directed to a method ofcompensating for birefringence in an image projection system. The methodincludes polarizing light at a polarizing layer substantially into aselected polarization state and directing the polarized light to animage-forming device through a birefringent element. The polarized lightis reflected at the image-forming device and the reflected polarizedlight is analyzed at the polarizing layer. The polarized light and thereflected polarized light are passed through a quarter-wave retardingelement disposed between the birefringent element and the image-formingdevice. The quarter-wave retarding element is oriented to substantiallyoptimize compensation of retardance in the birefringent element.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The following figures and detailed description moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates an embodiment of a projection systemaccording to principles of the present invention;

FIG. 2 schematically illustrates the operation of a polarizingbeamsplitter;

FIGS. 3A and 3B schematically illustrate experimental arrangements usedfor obtaining experimental results;

FIG. 4A schematically illustrates a projected image area;

FIGS. 4B and 4C present experimental results showing the ratio of thebrightness of the dark state light without quarter-wave compensation tothe brightness of dark state light with quarter-wave compensation fordifferent positions across the projected image area;

FIG. 5 schematically illustrates birefringence angle in the cover of apolarizing beamsplitter;

FIG. 6A schematically illustrates an embodiment of a polarizingbeamsplitter with a quarter wave retarder compensator, according toprinciples of the present invention;

FIGS. 6B-6D present graphs showing polarization angle for light atvarious points within the arrangement of FIG. 6A;

FIG. 7 presents a graph showing a calculated contour plot of contrast asa function of i) birefringence in the glass cover of a polarizingbeamsplitter and ii) orientation angle of the quarter-wave retarder, fora birefringence angle of 45°;

FIG. 8 presents a graph showing a calculated contour plot of contrast asa function of i) birefringence in the glass cover of a polarizingbeamsplitter and ii) orientation angle of the quarter-wave retarder, fora birefringence angle of 22.5°;

FIG. 9 presents a graph showing a calculated contour plot of contrast asa function of i) the birefringence angle and ii) the orientation of thequarter-wave retarder for a retardation of 10 nm in the glass cover; and

FIG. 10 presents a graph showing the contrast as a function ofbirefringence in the glass cover with and without compensation using aquarter-wave retarder.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to systems that use polarizingbeamsplitters (PBSs), and is believed to be particularly useful forimage projection systems that incorporate PBSs for separating imagelight generated using a polarization modulator from illumination light.While the invention may be useful in any application where a PBS isused, it is described below particularly as used in projection systems.The scope of the invention is not intended to be limited to onlyprojection systems.

The invention may be used in many different types of projection system.One exemplary embodiment of a multi-panel projection system 100 that mayincorporate the invention described below is schematically illustratedin FIG. 1. The projection system 100 is a three-panel projection system,having a light source 102 that generates a light beam 104, containinglight in three different color bands. The light beam 104 is split bycolor splitting elements 106 for example, dichroic mirrors, into first,second and third beams 104 a, 104 b and 104 c containing light ofdifferent colors. The beams 104 a, 104 b and 104 c may be, for example,red, green and blue in color respectively. Beam steering elements 108,for example mirrors or prisms, may be used to steer any of the beams104, 104 a, 104 b and 104 c.

The beams 104 a, 104 b and 104 c are directed to respective imageforming devices 110 a, 110 b and 110 c which may be, for example,LCD-based reflective image-forming panels, such as liquid crystal onsilicon (LCoS) panels. The light beams 104 a, 104 b and 104 c arecoupled to and from the respective image-forming devices 110 a, 110 band 110 c via respective polarizing beamsplitters (PBSs) 112 a, 112 band 112 c. The image-forming devices 110 a, 110 b and 110 c polarizationmodulate the incident light beams 104 a, 104 b and 104 c so that therespective image beams 114 a, 114 b and 114 c are separated by the PBSs112 a, 112 b and 112 c and pass to the color combiner unit 116. In theillustrated exemplary embodiment, the illumination light beams 104 a,104 b and 104 c are reflected by the PBSs 112 a, 112 b and 112 c to theimage-forming devices 110 a, 110 b and 110 c and the resulting imagelight beams 114 a, 114 b and 114 c are transmitted through the PBSs 112a, 112 b and 112 c. In another approach, not illustrated, theillumination light may be transmitted through the PBSs to theimage-forming devices, while the image light is reflected by the PBSs.

Quarter-wave retardation elements 111 a, 111 b, 111 c are positionedbetween the image-forming devices 110 a, 110 b, 110 c, and theirrespective PBSs 112 a, 112 b, 112 c. The quarter-wave retardationelements 111 a, 111 b, 111 c may be used for compensating for residualbirefringence in the image forming devices 110 a, 110 b, 110 c and, asis explained in greater detail below, for compensating birefringence inthe PBSs 112 a, 112 b, 112 c. In the illustrated exemplary embodiment,the color combiner unit 116 combines image light beams 114 a, 114 b and114 c of different colors, for example using one or more dichroicelements. In particular, the illustrated exemplary embodiment shows anx-cube color combiner, but other types of combiner may be used. Thethree image beams 114 a, 114 b and 114 c are combined in the colorcombiner unit 116 to produce a single, colored image beam 118 that maybe directed by a projection lens system 120 to a screen (not shown).

Other embodiments of projection systems may use one or more PBSs. Forexample, a projection system may use one or two image-forming devices,with respective PBSs, as is described in greater detail in U.S. patentapplication Ser. Nos. 10/439,449 and 10/914,596, incorporated herein byreference. The maximum number of image-forming devices is not limited tothree, and projection systems may use more than three image-formingdevices. In addition, different types of light sources may be used,including white light sources, such as high-pressure mercury lamps, andcolored light sources, such as light emitting diodes. The intention isnot to limit how the illumination light reaching the PBS is generated,or how the light is processed before reaching the PBS.

High quality polarizing beamsplitters (PBSs) for providing high contrastimages in projection systems have previously required the use of glasseshaving a low stress optic coefficient (SOC), also known as photoelasticconstant. Examples of low SOC glasses include PBH56 and SF57 glasses.These glasses have high lead content, for example 70% weight lead oxideor more, and a high refractive index, in excess of 1.8. The use of highquantities of lead in the PBS glass leads to environmental concerns. Inaddition, since the polarizing layers in a multilayer optical filmpolarizer have a refractive index typically in the range of about1.5-1.6, the refractive index difference between the polarizing layersand the glass is high, which may lead to a sub-optimal angle ofincidence on the polarizing layer when the low SOC glass is used. Thisrelatively large refractive index difference may lead to aberrationswhich can be addressed in the optical design of the system or the PBSitself, for example as discussed in U.S. Pat. Nos. 6,672,721 and6,786,604. Also, the low SOC glasses have a low Abbé number, which meansthat the dispersion is high, and so SOC glasses may be less suitable forapplications covering a wide range of wavelengths.

The elimination of lead from products is an important environmentalobjective, and one that affects the performance of birefringencesensitive optical systems. The present invention is directed to the useof standard glasses, for example N-BK7, N-SK5, and the like, availablefrom Schott North America, Duryea, Pa., or equivalent glasses. Forexample, S-BAL35 and ZK3 are roughly equivalent to NSK5, and arerespectively supplied by Ohara Incorporated, Japan and ChengduGuangming, China, respectively. Although these glasses have high valuesof SOC, for example more than up to one hundred times that of the lowSOC glasses listed above, they may be used according to the presentinvention in PBS applications that maintain high contrast, even whensubject to the stresses experienced in illuminating an imaging core forprojection televisions. These standard glasses do not contain lead.

Many unsuccessful attempts have been made to use lead-free glass inimaging PBS applications. For example, Cline et al. (Thermal stressbirefringence in LCOS projection displays, Display 23 (2002) pp.151-159) analyzed and tested a number of glasses forillumination-induced thermal stress birefringence, i.e. the stressbirefringence that arises due to absorptive heating of the glass whenilluminated by illumination light. The analysis produced a figure ofmerit for illumination-induced thermal stress birefringence, whichsuggested that the illumination-induced thermal stress birefringence ofSK5, BK7 and Ultran 30 might be sufficiently low for use in projectiontelevision systems. Cline et al. noted, however, that their analysis didnot take into account other sources of stress birefringence, includingmechanical stresses arising from assembly and mounting, and thermalstresses arising for reasons other than illumination, for examplecooling fans, heat from electronics or heat absorbed and radiated by thereflective imager. Some additional stress birefringence may arise, forexample, due to the adhesive used between the polarizing layer and theglass prism of the PBS. This adhesive may, for example, cure in such away as to produce stress in the glass cover, which leads to fabricationstresses. Furthermore, operation at elevated temperatures may lead to anadditional thermally induced stresses arising from different thermalexpansion coefficients among the adhesive, the glass cover and/or thepolarizing layer. These additional sources of stress induce significantstress birefringence, over and above the illumination-induced thermalbirefringence discussed by Cline, resulting in these non-leaded glassesbeing unsuited for simply replacing the low SOC glasses in a PBS used ina projection system. Experimental results are provided below that showthat the contrast of a PBS can be significantly compromised when usingN-SK5 glass.

Rather than eliminating the birefringence in a PBS component, someembodiments of the invention are directed to an approach forcompensating the effects of birefringent optical glass in a PBScomponent. While the invention is believed to be useful for manydifferent types of reflective polarizer layer in a PBS, it is believedto be particularly useful for Cartesian Multilayer Optical Film (MOF)PBSs, for example the polarizing films described co-owned U.S. Pat. No.6,486,997, incorporated herein by reference. The invention may also beeffective for the other types of PBSs, such as the MacNeille PBS and awire grid PBS, with a glass element between the polarizing layer and theimage-forming device.

The glasses that may be used in the present invention include glassesthat have a refractive index of less than 1.8, less than 1.7 and evenless than 1.6. In addition, the value of SOC for the glass in oneembodiment may be greater than 0.1×10⁻⁶ mm² N⁻¹, in another embodimentgreater than 0.5×10⁻⁶ mm² N⁻¹ or in another embodiment may be greaterthan 1.0×10⁻⁶ mm² N⁻¹. Some examples of lead-free glass that may be usedin the present invention to provide high contrast operation of a PBSinclude N-SK5 and N-BK7 glass, available from Schott North America,Duryea, Pa. N-SK5 glass has an SOC value of 2.16×10⁻⁶ mm² N⁻¹ in thevisible spectrum, and NBK-7 has an SOC value of 2.7×10⁻⁶ mm² N⁻¹. Thisis more than twenty times the SOC of PBH56 glass (0.09×10⁻⁶ mm² N⁻¹) andmore than one hundred times the SOC of SF57 glass (0.02×10⁻⁶ mm² N⁻¹).Consequently, the lead-free glasses can be about twenty to one hundredtimes more sensitive to mechanical and/or thermal stress than thehigh-lead glasses. A PBS assembled with no stress can still be sensitiveto the stresses developed by thermal excursions and the absorption ofmoisture in the adhesives and in the polymer films of the multilayerpolarizer. As is discussed below, even when a stress-free PBS wasprepared initially, it is difficult to use the PBS with a mirror darkstate and maintain good dark state uniformity as the PBS heats, cools,and is fixtured in place.

A quarter-wave retarding element, such as a quarter-wave film orquarter-wave plate, disposed between the PBS and the image-formingdevice, may be used to at least partially compensate for thestress-related birefringence in the lead-free glass. This reduces thedegradation in contrast usually associated with using conventional,lead-free, glasses in a PBS.

The birefringence of a stressed PBS is now described in greater detailwith reference to FIG. 2, which schematically illustrates a PBS 200 thathas a reflective polarizing layer 202 sandwiched between opposingsurfaces 204 a, 206 a of two covers 204, 206. The covers 204, 206 may bein the form of prisms, having additional surfaces that are non-parallelto the surfaces 204 a, 206 a for transmitting light into and out of thePBS 200. In the illustrated embodiment, the covers 204, 206 are rightangled prisms with the opposing surfaces 204 a, 206 a disposed at 45°relative to the output surface 204 b reflective polarizing layer 202,although the surfaces 204 a, 206 a may be disposed at different angles.The covers 204, 206 may be attached to the reflective polarizing layer202 using, for example, layers of adhesive 205 a, 205 b between thereflective polarizing layer 202 and the covers 204, 206 respectively.One suitable example of an adhesive is an optical epoxy as taught inU.S. Patent Publication No. 2004/0234774A1, incorporated herein byreference.

In some embodiments, the reflective polarizing layer 202 may be amultilayer optical film reflective polarizer, formed of alternatinglayers of different polymer materials, where one of the sets ofalternating layers is formed of a birefringent material: the refractiveindices of the different materials are matched for light polarized inone linear polarization state and unmatched for light in the orthogonallinear polarization state. Accordingly, incident light in the matchedpolarization state is substantially transmitted through the layer 202and the light in the unmatched polarization state is substantiallyreflected by the layer 202.

Other types of reflecting polarizer layer may be used, for example, astack of inorganic dielectric layers, as is often used in a MacNeillePBS, or wire grid polarizers used in glass prisms, as taught in U.S.Pat. No. 6,719,426, or any other polarization selective layer used in asimilar way. The reflective polarizing layer 202 may be cemented to thecovers 204, in a manner similar to that described above.

When unpolarized illumination light 210, or light of mixed polarizationstates, is incident on the PBS 200, the reflective polarizing layer 202substantially reflects light in one polarization state 210 a, referredto here as the s-polarization state, and substantially transmits theorthogonal polarization state, referred to here as the p-polarizationstate 210 b. It should be noted, however that, when the PBS is aCartesian MOF, the polarization states of the reflected and transmittedlight are determined in reference to fixed material axes of the PBS:they are determined by the direction of a physical anisotropy of thereflecting polarizing layer itself. For a MacNeille polarizer, thepolarization directions are determined with reference to the plane ofreflection at the polarizer. Accordingly, the terms s-polarization andp-polarization are here used to denote the orthogonal polarizationstates of light that are primarily reflected and transmitted by the PBSrespectively, both in the case of the MacNeille PBS, where thisnomenclature is strictly accurate, and in the case of Cartesian PBSs,where the nomenclature is approximately and substantially accurate, andremains convenient. Wire grid polarizer and MOF polarizers are bothexamples of Cartesian polarizers.

The fraction of the incident s-polarized light reflected by thereflective polarizing layer 202 is given as R_(s), and the fraction ofthe incident p-polarized light transmitted by the reflective polarizinglayer 202 is given as T_(p). Even if the reflective polarizing layer 202were to be a perfect polarizer, reflecting only s-polarized light(R_(s)=1) and transmitting only p-polarized light (T_(p)=1), the lightreaching the image forming device 212 is of mixed polarization, as isnow explained.

The first cover 204 is under stress, which results in stressbirefringence. The stress may arise for many different reasons. Forexample, the stress may result from mechanical considerations arisingduring the manufacturing process, or when the PBS 200 is mounted in theoptical system. In addition, the stress may be thermal in nature, forexample arising from illumination-induced thermal stress, or from adifference in the thermal expansion coefficients of the glass, theoptical adhesive, the reflective polarizing layer, or the PBS mount.Thus, even when no p-polarized light is reflected, i.e. T_(p)=1, thereflected light 210 a passes through the birefringent cover 204 to theimage-forming device 212, and so the light reaching the image-formingdevice 212 contains both s-polarized and p-polarized light. There mayalso be additional birefringence in the layer of adhesive 205 a betweenthe reflective polarizing layer 202 and the first cover 204.

The retarding effect due to the stress-induced birefringence may begreater at one side of the PBS 200 than the other, as is now explained.The light ray 220 passes into the left hand edge of the first cover 204,propagates along a relatively long path in the first cover 204, and isthen reflected as beam 220 a along a relatively short path within thefirst cover 204 to the image-forming device 212. The light ray 230passes into the right hand edge of the first cover 204, propagates alonga relatively short path within the first cover 204 and is then reflectedas beam 230 a along a relatively long path within the cover 204 to theimage-forming device 212. Thus, the light beam 230 a reaching theimage-forming device has passed through a longer path within the firstcover 204 after reflecting off the reflective polarizing layer 202 thanthe beam 220 a and, therefore, may contain a greater fraction ofp-polarized light than beam 220 a due to the birefringence in the firstcover 204. Light beams 220 b and 230 b, transmitted through thereflective polarizing layer 202, may be dumped.

In the dark state, the image-forming device 212 does not substantiallymodulate the polarization of the reflected light, and so thepolarization states of the light beams 222 and 232 reflected from theimage-forming device 212 are substantially the same as the lightincident at the image-forming device 212. Since beam 222 contains littlep-polarized light, only a relatively small amount of light istransmitted by the reflective polarizing layer 202 as beam 222 a, and arelatively large amount of light is reflected as beam 222 b. Beam 232 onthe other hand, contains a larger fraction of p-polarized light, whichincreases on passing back to the reflective polarizing layer 202, and sothe light transmitted as beam 232 a may have a greater intensity thanbeam 222 a. Accordingly, beam 232 b, reflected by the reflectivepolarizing layer 202, may be less intense than beam 222 b. Thus, in thedark state, the image light may be non-uniform, and be brighter on oneside of the PBS relative to the other.

The effect of stress birefringence in the first cover 204 applies to alllight rays incident on the image-forming device, including on-axis raysand skew rays. Furthermore, the effect increases from one side of thePBS to another, and is, therefore, not symmetrical about the center ofthe illumination light beam, at least in the plane of reflection.

This behavior has been observed experimentally. Two experimental set-upswere used, shown in FIGS. 3A and 3B. In the set-up 300 shown in FIG. 3A,s-polarized light 302 from a high pressure, mercury arc lamp, wasdirected in an f/2.3 light beam to a MOF PBS 304. The MOF PBS 304reflected substantially s-polarized light to a plain mirror 306, whichwas used to emulate a reflective image-forming device in the dark state.The light 308 that was reflected from the mirror 306 and transmittedthrough the PBS 304 was directed via a turning prism 310 to a lens 312that formed an image of the mirror 306 on a 50″ (127 cm) diagonalscreen. The image on the screen was then captured using a ProMetricPM-1421-1 Imaging Colorimeter made by Radiant Imaging Inc. (DuvalWash.). The PBS was an MOF PBS, as described in co-owned U.S. Pat. Nos.6,609,705, and 6,721,096, incorporated herein by reference. The PBS hada polarizing layer with values of both Tp and Rs exceeding 95% over thevisible region (430 nm to 700 nm) for the f/2.3 light beam.

The set-up 350 shown in FIG. 3B is the same as that of FIG. 3A exceptthat the mirror 306 was replaced with a quarter-wave mirror (QWM) 356,i.e. a mirror 357 with a quarter-wave retarder 358 attached. Thedark-state image was recorded for each set-up 300, 350. The QWM 356 wasrotated about an axis parallel to the z-axis to obtain the best darkstate. A bright-state image was also recorded for the QWM set-up 350, byrotating the QWM 356 about an axis parallel to the z-axis to obtain thebrightest beam at the screen.

FIG. 4A schematically shows the outer perimeter of the projected image400, having a 16:9 aspect ratio. The six dashed lines 402 a-c, 404 a-c,represent lines across the projected image. FIG. 4B shows the brightnessof the mirror dark state along the three vertical lines 402 a-c on thescreen: curve 412 a shows the dark state brightness along line 402 a,curve 412 b shows the dark state brightness along line 412 b and curve412 c shows the dark state brightness along the line 402 c. The valuesof the dark state brightness are normalized to the QWM dark statebrightness, i.e. the values in the graph were obtained by dividing themirror dark state brightness by the QWM dark state brightness. Thisremoved some systematic artifacts from the results, for example arisingfrom any nonuniformity in the intensity profile of the beam illuminatingthe mirrors.

The co-ordinate system places the origin at the center of the screen,where lines 402 b and 404 b cross. The vertical lines 402 a-c haverespective x-coordinates of −0.45, 0 and 0.45, and the horizontal lines404 a-c have respective y-coordinates of 0.27, 0 and −0.27. Therefore,the points marked A-I, the crossing points of the lines 402 a-c and 404a-c, have the coordinates as follows A (−0.45, 0.27), B (0, 0.27), C(0.45, 0.27), D (−0.45, 0), E (0, 0), F (0.45, 0), G (−0.45, −0.27), H(0, −0.27), I (0.45, −0.27). The coordinates correspond to positions (inmeters) measured on the projection screen.

Looking first at curve 412 c, corresponding to the right side of theimage, line 402 c, the mirror dark state brightness is close to the QWMbrightness for positions between about −0.3 m and zero m. However, forpositions between zero m and 0.3 m, the relative brightness of themirror dark state increases dramatically, up to a value of about 7. Inother words, the mirror dark state is about seven times brighter thanthe QWM dark state. This means that there is significantly more darkstate leakage at this region when there is no quarter wave retarderpresent. Thus, the top right corner of the image appeared brighter thanthe bottom left corner. The other curves 412 a, 412 b do not vary asmuch as curve 412 c over the height of the image, showing that the darkstate image is brightest in the top right corner of the image 400.

FIG. 4C shows the normalized brightness of the mirror dark state for thethree horizontal lines 404 a-c across the screen 400: curve 414 a showsthe dark state brightness along line 404 a, curve 414 b shows the darkstate brightness along line 414 b and curve 414 c shows the dark statebrightness along the line 404 c. Curve 414 a, showing the dark stateimage brightness for positions along the top of the image 400, hasrelatively low values for positions to the left side of the screen(positions less than zero m) but is high for positions to the right sideof the screen. This is in agreement with the results discussed in theprevious paragraph for the vertical lines 412 a-c. Curves 414 b and 414c are relatively flat, indicating that there is not much difference inthe dark state intensity for the mirror 306 or the QWM 356 in theseareas. The points marked 416 correspond to areas where the presence ofdust prevented a realistic measurement of the dark state brightness.

The contrast ratio, the ratio of the bright state illuminance over thedark state illuminance, was calculated for nine different locationswithin the projected image area, corresponding to positions A-I. Thecontrast ratio was measured by measuring the brightness of the darkstate light and the brightness of the brightest state when the QWM wasrotated to a condition giving maximum transmission to the detector 314.The results of the contrast measurements for each of the set-ups 300 and350 are summarized in Table I.

TABLE I Contrast Ratios for various locations in the image light withand without quarter-wave compensation Dark State A B C D E F G H IMirror (306) 15214 2397 1051 7972 4294 6904 5926 6661 19171 QWM (356)6052 5009 6844 8275 5754 6252 7723 8223 11486

It is preferred that the contrast ratio for television projectionsystems be at a level of 2500 or above. As can be seen, points B and Cdo not meet this criterion when the mirror 306 alone was used: this isdue to stress birefringence in the PBS cover. In contrast, the contrastratio for the QWM dark state is well above 2500 for all points on theprojected image.

For some points, for example points A and I, the contrast issignificantly higher when the mirror 306 was used, rather than the QWM356. These correspond to positions in the PBS cover that suffer fromvery little, or no stress birefringence. The resulting contrast ratio isreduced for the QWM projected image because of imperfections in thequarter-wave retarder 358, for example imperfections in orientation,possibly induced by mounting or inherent in the retarder 358 itself. Inaddition, the setup 350 using the QWM 356 was not wavelength optimized,and improved contrast performance is expected with optimized components.The quarter-wave retarder 358 on the QWM 356 was not spectrally neutral,but rather had a birefringence of about 137 nm. Therefore, thequarter-wave film only provided true quarter-wave retardation forwavelengths near 550 nm. Improved contrast results would be expected inthe QWM case by narrowing the bandwidth of the incident light used or byusing a QWM 356 having a flatter retardation over the wavelength rangeof the illumination light. Note that, in 3-panel projection systems, thebandwidth of the light incident on the image-producing device istypically about 70 nm or less, and the optical elements, including thePBS and the quarter-wave retarder, may be optimized for operation ineach projected color band.

The effect of using a quarter-wave retarder (QWR) has been modeledutilizing Mueller Matrix polarization modeling. The simplest model thatdemonstrates the effect includes crossed linear polarizers for theequivalent function of the PBS function: crossed polarizers areequivalent to one reflection and one transmission through the PBS.Between the equivalent polarizers are placed birefringent glass, tomodel the PBS cover, a QWR, an LCoS imager with some stray retardance,and a mirror.

FIG. 5 schematically illustrates a face (500) of a PBS, showing thedirection of the polarization (s-pol), and various arrows in the face500 showing different directions of birefringence. The direction of thebirefringence is dependent on the direction of the stress in the glassmaterial. The resulting analysis shows that a QWR compensatorsubstantially corrects any level of birefringence, or stress, orientedat 45° to the polarization direction, the arrow labeled (a). Stressoriented at 0° or 90° (arrows (b) and (c) respectively) has no effect onthe polarization of the polarized light beam. The analysis indicatesthat stress oriented at 22.5° (modulo 45°) (arrows (d) and (e)) is themost difficult to compensate with the QWR.

The reasons for this behavior are now discussed with reference to FIGS.6A-D. FIG. 6A schematically illustrates a light beam 610 entering a PBS600 formed by a reflective polarizing layer 602 between two covers 604,606. The light enters the first cover 604, and reflects off thereflective polarizing layer 602 towards the image-forming device 608.The first cover 604 displays stress birefringence.

The light 610 is polarized, or repolarized if it had previously beenprepolarized, at the reflective polarizing layer 602. This assumes thatthe reflective polarizing layer 602 has high values of T_(p), at leastover 90% across the bandwidth of the incident light and range ofincident angles, preferably over 92% and more preferably over 94%. Themultilayer film polarizing layers described in U.S. Pat. No. 6,609,795achieve high values of T_(p), reaching typical values of between 96% and98% for an f/2.3 beam over the entire wavelength range of 430 nm to 700nm. Thus, only a small faction of the light in the unwanted polarizationstate is reflected to the image-forming device 608.

The reflected light 610 a propagates along a path through the firstcover 604. Upon exiting the first cover 604, the polarization state ofthe light 610 a at plane A, a plane perpendicular to the z-axis, is acombination of orthogonally polarized components. An exemplary selectionof polarization components for the light beam is shown in FIG. 6B, whichshows the polarization state of a portion of beam 610 a passing througha region of glass retardation, as seen by a viewer looking through thePBS 600 at the image-forming device, along the negative z direction. Thes-polarization direction is assumed to be parallel to the y-directionand the p-polarization direction is assumed to be parallel to thex-direction, as would be the case for an axial ray. The light beam 610has a polarization angle of θ₀ relative to the y-axis (s-polarizationdirection). The light is elliptically polarized and the polarizationangle, θ₀, is the direction of the major axis of the polarizationellipse. The polarization angle θ₀, for retardation at 45° to thepolarization direction, is obtained using the expression:

θ₀=tan⁻¹(I _(x) /I _(y)),

where I _(x) =I _(o)·sin²(2πΔn·d/λ), and

I _(y) =I _(o)·cos²(2πΔn·d/λ).

I₀ is the intensity of the incoming beam, I_(x) and I_(y) are therelative intensities for light polarized parallel to the x-axis(p-polarized light) and light polarized parallel to the y-axis(s-polarized light), respectively, Δn·d is the total summed retardationalong the light path in the region of glass under consideration, and λis the wavelength of the light in vacuum. For other orientations of theretardation the equation is more complex, but qualitatively similar. Theactual value of θ₀ depends on the retardance of the first cover 604 and,in practical situations, may have a value less than that illustrated inFIG. 6B. The particular value of θ₀ illustrated in FIG. 6B was selectedfor illustrative purposes only.

The light beam 610 a passes through the through the QWR 612 and reflectsoff the image-forming device 608 as reflected beam 614 and passes oncemore through the QWR. The double-pass through the QWR 612 isfunctionally equivalent to passing through a half-wave retarder. As aresult, the polarization angle of the reflected beam 614 is flipped to−θ₀. FIG. 6C shows the polarization angle of the reflected beam 614 atthe plane A, after reflecting from the image-forming device 608.

After passing back through the QWR 612, the light beam 614 followsapproximately the same path through the cover 604 to the reflectivepolarizing layer 602 and, therefore, experiences substantially the sameretardation due to stress birefringence as the light 610 did on theincoming path. Since the polarization angle of the light beam 614 isinverted relative to the light beam 610 at plane A, however, the neteffect is that, once the beam 614 has passed back through the firstcover 604 to the polarizing layer 602, the polarization angle, θ, hasbeen rotated back to a value at, or close to, zero. This isschematically illustrated in FIG. 6D, which shows the polarization stateof the light beam 614 at plane B, at the reflective polarizing layer602. In the illustrated example, the compensation is ideal and the lightbeam 614 is s-polarized, parallel to the y-axis. This light will bereflected by the polarizing layer 602 of the PBS 600 and does notcontribute substantially to the brightness of the projected image, asshould be the case for a dark state image. The polarization-modulatedlight 614, on the other hand, passes through the reflective polarizinglayer 602 as image light. It will be appreciated that compensation forthe birefringence of the PBS cover may not always be complete, forreasons discussed below. However, the degree of compensation can besignificant, and permits lead-free glasses to be used in PBSs with highcontrast.

The compensation of stress birefringence using a QWR 612 works well forlight 610 a that is on-axis, i.e. light that propagates through thefirst cover 604 from the reflective polarizing layer 602 in a directionparallel to the z-axis, and where the reflective polarizer has a valueof T_(p) that is relatively high. In practice, however, the conditionsare less advantageous. For example, an image-forming element istypically illuminated with light that falls within a specified coneangle and, as a result, the light rays incident at the image-formingdevice 608 are not necessarily parallel to the z-axis. Thus, thebirefringence history for the incoming and outgoing light rays becomesdifferent: the rotation of the polarization angle for the illuminationlight is increasingly different from the rotation of the polarizationfor the reflected light rays propagating at increased angles from thez-axis. Thus, the extent to which the stress birefringence in the firstcover 604 is compensated may be reduced.

Furthermore, as the value of T_(p) drops below 100%, the light reflectedby the reflective polarizing layer 604 towards the image-forming device608 contains an increasing amount of p-polarized light. Only thebirefringence effects after reflection from the polarizing layer 602, onthe optical path between the polarizing layer 602 and the image-formingdevice 608, are compensated by the polarization angle inversion due tothe QWR 612. Accordingly, to the extent that the reflected light 602 acontains p-polarized light, the contrast will be degraded. Thus, it isvery important that the T_(p) be high, so as to reduce the amount ofp-polarized light reflected at the reflective polarizing layer 602. Thisapproach to reducing the effects of stress birefringence is most useful,therefore, when the value of T_(p) is higher than 90% for extreme rayswhen using illumination light having a useful f-number, say f/3.0 oreven down to f/2.0 or less. Preferably, the value of T_(p) is higherthan 92% and more preferably higher than 94%.

The QWR 612 at least partially compensates for birefringence in anycomponent elements, in addition to the PBS cover 604, located betweenthe polarizing layer 602 and the QWR 612. For example, the QWR 612 maycompensate for birefringence arising in an adhesive layer between thecover 604 and the polarizing layer 602. In addition, an optional fieldlens 618 may be positioned close to the image-forming device 608: thisarrangement gains additional design freedom for the projection lens,especially in terms of lateral color. Birefringence in the field lens618 may be compensated by the QWR 612 if the field lens 618 ispositioned between the QWR 612 and the polarizing layer 612. Othercomponent elements that might be positioned between the QWR 612 and thepolarizing layer 602 include, for example, a glass substrate (not shown)for supporting the QWR 612.

The optical surfaces between the mirror surface of the image-formingdevice 608 and the polarizing layer 602 are may be provided withantireflection properties, for example using an antireflection coating.This reduces reflection losses, thus increasing optical throughput inthe image light. Also, the use of antireflection surfaces may reduce theoccurrence of haloes around bright areas set against a dark backgroundin a projected image. Such haloes may arise from rays diverging from thebright area off an optical surface back to the mirror in theimage-forming device 608. The light then reflects off the mirror withthe correct polarization state to pass through the PBS 600 as imagelight. These rays lie outside the bright area and appear to the viewerto originate from the dark area immediately surrounding the bright area.

Some results from the numerical modeling are now discussed. FIG. 7 showsa contour plot showing the calculated contrast ratios as a function ofi) retardance in the glass cover 604 for a single-pass through the glasscover, between the polarizing layer 602 and the image-forming device608, and ii) the orientation of the QWR 612. The orientation of the QWR612 is provided in degrees relative to the condition where the fast axisof the QWR 612 is parallel or perpendicular to the plane of reflection.The birefringence in the cover 604 was assumed to be at 45° relative tothe plane of reflection. The model assumed that the image-forming deviceis a VAN-mode liquid crystal display panel with an in-plane residualbirefringence of 5 nm at 45° relative to the plane of reflection. As canbe seen, the maximum contrast is achieved when the QWR 612 is rotatedwith its fast axis at about 1.8° relative to the plane of reflection.This calculation assumed a value of Tp=100% in the PBS 600. Since someMOF PBSs may demonstrate a value of Tp exceeding 98%, this assumption isnot unreasonable. Note that, as discussed above, any amount ofbirefringence in the glass cover 604 is exactly compensated with onesingle orientation of the QWR 612 for this orientation of birefringencein the glass cover 604.

FIG. 8 shows a contour plot similar to that of FIG. 7, but for thebirefringence in the glass cover 604 oriented at 22.5°, which is themost difficult condition to compensate using this approach. It is clearthat, at some level of retardation, the QWR will not perfectlycompensate the birefringence. However, judicious selection of theorientation angle of the QWR, for example at the level indicated by thestraight line 802, still provides for a relatively high contrast ratio,in excess of 6000:1, for a glass cover retardance up to 20 nm. It shouldbe noted that, in a practical PBS, the birefringence in a glass cover isnot all oriented at 22.5° and, hence, the actual contrast ratioobtainable may be higher than shown in this graph. Thus, the QWR may beused to compensate for levels of retardation in the cover that aregreater than 1 nm, greater than 5 nm and greater than 10 nm.

FIG. 9 shows a contour plot of contrast as a function of the orientationangle of the birefringence in the cover 604 and orientation of the QWR612. The results in this plot were obtained under the assumption thatthe retardation of the glass cover 604 was 10 nm. These results clearlyshow that a single orientation of the QWR 612, as indicated by thehorizontal line 902, provides high contrast performance for any angle ofbirefringence orientation, in excess of 8000:1.

FIG. 10 presents a graph that compares the contrast as a function ofretardation in the glass cover 604 for a) no QWR compensation, curve1002, b) optimized QWR compensation, curve 1004, assuming no residualbirefringence in the image-forming device and c) optimized QWRcompensation, curve 1006, where there is about 5 nm of residualretardation at 45° to the incoming polarization state in the off-state.The orientation of the birefringence in the glass cover 604 was assumedto be 25°. Without compensation, curve 1002, the contrast drops rapidlyat small values of retardation in the glass cover 604. At about 0.004waves of retardation (˜2 nm for visible light), the contrast is reducedby half. In the compensated cases, however, curves 1004, 1006, theretardation magnitude must exceed 0.03 (˜15 nm for visible light) tocause a similar level of contrast reduction. Useful PBS contrast levelsshould exceed 2500:1 for most rear-projection television (RPTV). Theresults in FIG. 10 show that this level of contrast requires that therebe no more than 0.004 waves of retardation at a stress orientation of25°, when there is no QWR compensation. In comparison, thebirefringence-compensated system can tolerate at least ten times thislevel of birefringence, while still meeting the contrast requirement of2500:1. From the experimental results summarized in Table I, it can beestimated that the glass cover has a stress-induced retardation of about0.0085 waves (about 4.7 nm for visible light), which is more than theuncompensated system can tolerate, but well within the levels ofretardation acceptable with a compensated system.

Another factor that may be considered in reducing the effects ofstress-induced birefringence, in addition to compensating with a QWR, isto reduce the amount of stress applied to the PBS. Some sources ofstress, such as thermally induced stress arising from optical absorptionof the light in the PBS glass, are difficult to avoid. Other sources ofstress, for example stresses arising from mounting may be reducedthrough careful engineering and design.

It is often convenient to provide an optical core comprising the PBS, orPBSs, attached to an optical element. For example, the projection system100 of FIG. 1 shows the PBSs 112 a, 112 b, 112 c attached to the colorcombiner unit 116, for example using an optical adhesive such as apressure sensitive adhesive or optical epoxy.

Accordingly, the present invention should not be considered limited tothe particular examples described above, but rather should be understoodto cover all aspects of the invention as fairly set out in the attachedclaims. Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An optical system, comprising: a first polarizing beamsplitter (PBS)having a reflective polarizing layer disposed between first and secondcovers, the first cover being formed of a material having a stress opticcoefficient greater than 0.1×10⁻⁶ mm² N⁻¹; a first reflectingimage-forming device facing a first surface of the first cover so thatlight passes through the first cover from the reflective polarizinglayer of the first PBS to the first reflecting image-forming device andis reflected from the reflecting image-forming device through the firstcover to the reflective polarizing layer of the first PBS, the lightbeing subject to stress birefringence in the first cover and to residualbirefringence in the reflecting image-forming device; and a firstquarter-wave retarding element disposed to retard the light passingbetween the first surface of the first cover and the reflectingimage-forming device.
 2. A system as recited in claim 1, wherein thefirst quarter-wave retarding element is oriented to maximizecompensation for the birefringence in the first cover of the first PBSand the reflective image-forming device.
 3. A system as recited in claim1, wherein the stress optic coefficient is greater than 0.5×10⁶ mm² N⁻¹.4. A system as recited in claim 3, wherein the stress optic coefficientis greater than 1.0×10⁻⁶ mm² N⁻¹.
 5. A system as recited in claim 1,wherein the first cover is formed of a glass having a refractive indexno greater than 1.8.
 6. A system as recited in claim 5, wherein theglass has a refractive index of no greater than 1.7.
 7. A system asrecited in claim 6, wherein the glass has a refractive index no greaterthan 1.6.
 8. A system as recited in claim 1, further comprising a lightsource and beam management optics to direct an illumination light beamto the image-forming device via the PBS.
 9. A system as recited in claim1, further comprising a projection lens unit arranged to project animage from the image-forming device.
 10. A system as recited in claim 9,wherein the projected image has a contrast ratio of no less than 1000:1.11. A system as recited in claim 10, wherein the contrast ratio is noless than 2500:1.
 12. A system as recited in claim 11, furthercomprising at least a second PBS, at least a second reflectingimage-forming device and a color combining unit, image light from thefirst and at least a second reflecting image-forming device beingcombined in the color combining unit to produce combined image beam. 13.A system as recited in claim 12, further comprising at least a third PBSand at least a third reflecting image-forming device, image light fromthe first, second and at least a third image-forming device beingcombined in the color combining unit to produce the combined image beam.14. A system as recited in claim 13, wherein the color combining unitcomprises an x-cube color combining unit, the first, second and thirdPBSs being attached to the x-cube color combining unit, the first,second and third reflecting image-forming devices being attached to thefirst, second and third PBSs respectively.
 15. An optical system,comprising: an image-forming device; a reflective polarizing layerdisposed to reflect light in a selected polarization state to or fromthe image forming device; at least a first birefringent element disposedon an optical path between the image-forming device and the polarizinglayer, the first birefringent element having a face substantiallyperpendicular to the optical path, the birefringence of the birefringentelement being non-uniform so that values of birefringent retardation forlight passing through different portions of the face are different; anda quarter-wave retarding element disposed between the first birefringentelement and the image-forming device, the quarter-wave retarding elementoriented to at least partially compensate retardation of light on around trip between the reflective polarizing layer and the image-formingelement.
 16. A system as recited in claim 15, wherein the firstbirefringent element comprises a glass cover for the reflectivepolarizing layer.
 17. A system as recited in claim 15, wherein the firstbirefringent element has a single pass retardance in excess of at least1 nm for light passing through at least one portion of the face.
 18. Asystem as recited in claim 17, wherein the first birefringent elementhas a single pass retardance in excess of at least 5 nm.
 19. A system asrecited in claim 18, wherein the first birefringent element has a singlepass retardance in excess of at least 10 nm.
 20. A system as recited inclaim 15, wherein the first birefringent element is a glass cover havinga stress optic coefficient greater than 0.5×10⁻⁶ mm² N⁻¹.
 21. A systemas recited in claim 20, wherein the stress optic coefficient is greaterthan 1.0×10⁻⁶ mm² N⁻¹.
 22. A system as recited in claim 15, furthercomprising a second birefringent element disposed on the optical pathbetween the image-forming device and the polarizing layer.
 23. A systemas recited in claim 22, wherein the second birefringent elementcomprises a lens.
 24. A system as recited in claim 15, furthercomprising a light source and beam management optics to direct anillumination light beam to the image-forming device via the reflectivepolarizing layer.
 25. A system as recited in claim 15, furthercomprising a projection lens unit arranged to project an image from theimage-forming device.
 26. A system as recited in claim 25, wherein theimage has a contrast ratio of no less than 2500:1.
 27. An opticalsystem, comprising: a color combiner unit having at least two entrancefaces associated with different color bands; at least two polarizingbeamsplitters (PBSs) operatively disposed relative to the at least twoentrance faces respectively, at least a first PBS of the at least twoPBSs comprising a first reflective polarizing layer disposed betweenrespective first surfaces of first and second covers, the second coverpositioned between the reflective polarizing layer and the colorcombiner unit, the first cover being formed of a transparent materialhaving a stress-optic coefficient (SOC) greater than 0.1×10⁻⁶ mm² N⁻¹,the first PBS defining a first plane of reflection; and a firstquarter-wave retarding element, the first cover of the first PBS betweenthe quarter-wave retarding element and the reflective polarizing layer,the quarter-wave retarding element having a fast axis orientednon-parallel and non-perpendicular to the first plane of reflection. 28.A system as recited in claim 27, wherein a second PBS of the at leasttwo polarizing beamsplitters comprises a second reflective polarizinglayer disposed between respective first surfaces of third and fourthcovers, the fourth cover being positioned between the second reflectivepolarizing layer and the color combiner unit, the third cover beingformed of a transparent material having a stress-optic coefficient (SOC)greater than 0.1×10⁻⁶ mm² N⁻¹, the second PBS defining a second plane ofreflection, a second quarter-wave retarding element, the third cover ofthe second PBS lying between the second quarter-wave retarding elementand the second reflective polarizing layer, the second quarter-waveretarding element having a fast axis oriented non-parallel andnon-perpendicular to the second plane of reflection.
 29. A system asrecited in claim 28, wherein a third PBS of the at least two polarizingbeamsplitters comprises a third reflective polarizing layer disposedbetween respective first surfaces of fifth and sixth covers, the sixthcover being positioned between the third reflective polarizing layer andthe color combiner unit, the fifth cover being formed of a transparentmaterial having a stress-optic coefficient (SOC) greater than 0.1×10⁻⁶mm² N⁻¹, the third PBS defining a third plane of reflection, a thirdquarter-wave retarding element, the fifth cover of the third PBS lyingbetween the third quarter-wave retarding element and the thirdreflective polarizing layer, the third quarter-wave retarding elementhaving a fast axis oriented non-parallel and non-perpendicular to thethird plane of reflection.
 30. A system as recited in claim 29, whereinthe first, second and third PBSs are attached to the color combinerunit.
 31. A system as recited in claim 30, wherein the second, fourthand sixth covers are attached to the color combiner unit.
 32. A systemas recited in claim 29, further comprising respective first, second andthird image-forming devices operatively disposed respectively proximatethe first, second and third PBSs.
 33. A system as recited in claim 32,wherein the first, second and third image-forming devices are mounted tothe second, fourth and sixth covers respectively.
 34. A system asrecited in claim 32, wherein at least one of the first, second and thirdimage-forming devices is a reflective image-forming device.
 35. A systemas recited in claim 27, further comprising a light source and lightmanagement optics arranged to direct illumination light to the firstPBS.
 36. A system as recited in claim 27, further comprising aprojection lens unit arranged to project an image received from thecolor combiner unit.
 37. A method of compensating for birefringence inan image projection system, comprising: polarizing light at a polarizinglayer substantially into a selected polarization state; directing thepolarized light to an image-forming device through a birefringentelement; reflecting the polarized light at the image-forming device;analyzing the reflected polarized light at the polarizing layer; andpassing the polarized light and the reflected polarized light through aquarter-wave retarding element disposed between the birefringent elementand the image-forming device, the quarter-wave retarding elementoriented to substantially optimize compensation of retardance in thebirefringent element.
 38. A method as recited in claim 37, furthercomprising generating the light and directing the light to thepolarizing layer.
 39. A method as recited in claim 37, furthercomprising projecting the analyzed light to a screen.